Geochemical Characteristics And Geodynamic Processes Of The Late Mesozoic Granitoid In The Eastern Segment Of The Yangtze Block (Northern Zhejiang Province)

Although Mesozoic granites are widely distributed in Southeast China,their petrogenesis and geodynamic settings are still matters of dispute.This paper reports the zircon U-Pb dating and Hf isotopes,whole-rock major and trace elements,whole-rock Sr-Nd and Sr-Pb isotopic compositions of feldspar of twenty late Mesozoic plutons from Chishi,Zhongtai,Zujianshan,Machebu,Shenzhongwu,Daixi,Kangshan,Shunxi,Miaoxi,Daochang,Moganshan,Tingzishan,Liuchun,Shanchuanxiang,Huangfenwu,Xianxia,Tangshe,Tonglizhuang,Yangshan and Gaoling in northern Zhejiang Province from the Eastern Segment of the Yangtze Block were analyzed to investigate their petrogenesis.Combined previous published date,we have known space and time distributions and petrogenesis of the late Mesozoic magmatic rocks in the region,and to reveal the corresponding geodynamics process responsible for the late Mesozoic tectonic evolution in the northern Zhejiang Province from the Eastern Segment of the Yangtze Block.We compared the geochemical characteristics of the late Mesozoic orebearing and ore-barren rocks and provide some clues for mineralization mechanism.The main results obtained are as follows:(1)The results of U-Pb zircon dating for Late Mesozoic Granitoid in the Eastern Segment of the Yangtze Block indicated the formation of their age ranging from 124 to 152 Ma.They can be divided into: the Early Yanshannian(Jurassic,> 145 Ma)and the Late Yanshanian(Cretaceous,< 145 Ma).The Early Yanshannian(Jurassic)exposed rocks are less,and mainly are granite and granodiorite.Alternatively,the Late Yanshannian(Cretaceous)rocks are very widely distributed and are mainly A-type granites and I-type granites distributed along the NE-trending fault zone,which have more rock categories.(2)The Jurassic Chishi pluton is the diorite,and Zircon U–Pb dating yields weighted mean 206Pb/238 U age of 147 ~152 Ma.They are featured by low Si O2(52.89 ~ 69.61 wt.%)contents,relatively high V(32.1 ~ 197 ppm),Cr(2.01 ~ 23.2 ppm)and Ni(2.31 ~ 11.3 ppm)contents and relatively high Mg#= 39 ~ 45.The Chishi pluton is metaluminous and peraluminous rock.They show light rare earth element(LREE)-enriched REE patterns((La/Yb)N=6.38 ~ 15.12)with weakly positive and negative Eu anomalies(?Eu=0.71 ~ 1.17)and are featured by enrichments in Ba,Pb and Sr and remarkable depletions in Nb,Ta,Zr,Hf and Ti.Combined enriched Sr-Nd-Hf isotopic compositions((87Sr/86Sr)i=0.7059 ~ 0.7070,?Nd(t)=-3.5 ~-3.3 and zircon ?Hf(t)=-16.7 ~-3.3))and two Neoproterozoic inherited zircons(628 ± 7.1 Ma(?Hf(t)=-0.2)and 765 ± 15.2 Ma(?Hf(t)=-13.1)),we suggest that the Jurassic Chishi pluton could have been derived from the Neoproterozoic basaltic rocks and late assimilation contamination and crystallization differentiation were not obvious.(3)The Zhongtai and Zujianshan plutons are the granodiorite,and zircon U–Pb dating yield weighted mean 206Pb/238 U ages of 140 ~ 146 Ma.The Zhongtai granodiorite is metaluminous and weakly peraluminous rock.The Zujianshan pluton is peraluminous rock.They show light LREE-enriched REE patterns((La/Yb)N=4.63 ~ 10.34)with weakly negative Eu anomalies(?Eu=0.57 ~ 0.83)and are featured by enrichments in Ba,U,Pb and Sr and depletions in Nb,Ta,and Ti.Combined Sr-Nd-Hf isotopic compositions((87Sr/86Sr)i=0.7059 ~ 0.7071,?Nd(t)=-3.8 ~-3.5 and zircon ?Hf(t)=-9.5 ~-0.6)),Two-stage Nd model(TDM2(Nd))ages(1056 ~ 1077 Ma)and Twostage Hf model(TDM2(Hf))ages(1028 ~ 1586 Ma),we suggest that the Zhongtai and Zujianshan plutons could have been derived from the Neoproterozoic intermediate and acidic rocks.The Zhongtai rock related to Au and Ag mineralization has the characteristics of high oxygen fugacity.The Machebu A-type granite and the Daixi highly fractionated I-type granite have the characteristics of low oxygen fugacity,related to Sn and W-Be mineralization,respectively.(4)The Machebu,Shenzhongwu,Daixi and Kangshan plutons are featured by an A2-type granite geochemical signature.Their zircon U–Pb dating yield weighted mean 206Pb/238 U ages of 127 ~ 135 Ma.They and other reported A-type granites form a NEtrending A-type granite belt along the Gan-Hang Belt.The formation ages of these Atype granites cluster in the range of 122 ~ 136 Ma and concentrate on 130 Ma.They are weakly peraluminous rocks and show high Si O2(71.06 to 77.73 wt.%),total alkalis(Na2O + K2O=7.57 to 9.12 wt.%),rare earth elements(total REE = 174 to 519 ppm)and HFSE contents,with mostly high Fe OT/(Fe OT + Mg O)(0.82 to 0.93)and Ga/Al ratios(2.49 to 5.07)and low Ce4+/Ce3+ ratios in zircons(1 to 90)),suggesting they were formed in an extensional tectonic setting.They show LREE-enriched REE patterns((La/Yb)N=1.54 ~11.04)with extremely negative Eu anomalies(?Eu=0.01 ~ 0.25)and are featured by enrichments in Rb,Th,U and Pb and remarkable depletions in Ba,Sr,Eu and Ti.Combining whole rock Zr contents,mineral assemblages and the An content of plagioclase,it was suggested that these granites could have formed at a high temperature(>850 °C)with a low H2 O content(~2.5 wt.%).Assuming the granite with the lowest Si O2 and high Ca O contents as the “primary granite melt”,the melting pressure was estimated to be 2.5 kbar based on model calculations using MELTs.Taking into account the effect of plagioclase fractional crystallization during the granite emplacement into shallow crustal levels,the initial melting pressure could be >2.5 kbar at the stable field of plagioclase.The results of in situ-Pb isotopic compositions of Kfeldspars show that the melting sources of these A-type granites are mainly continental lower crust of Yangtze Block.The ?Hf(t)values of the zircons in these plutons are-14.5 ~-3.5 for the Machebu granite,-1.5 ~-0.2 for the Shenzhongwu granite,5.1 ~ 8.6 for the Daixi granite,-25.8 ~-8.2 for the Kangshan granite,suggesting the origin of different parent rocks.These granites have two-stage Nd model(TDM2(Nd))ages of 1099 Ma ~ 838 Ma,and zircons from these granites and the Neoproterozoic basement in the Gan-Hang Belt plot on the same evolutionary trend in the ?Hf(t)-age diagram.It is interesting to note that the collection of literature data shows a positive correlation between Si O2 and ?Nd(t)for the late Mesozoic A-granites in the Gan-Hang Belt,and the Neoproterozoic A-granites in the Gan-Hang Belt cluster in two groups of the high-Si O2-?Nd(t)group and low-?Nd(t)group.The positive correlation of Si O2-?Nd(t)demonstrated by the late Mesozoic A-granites can be well explained by a high-degree of melting of mixtures between the two groups of Neoproterozoic A-granites.We thus suggest that the late Mesozoic A-type granites in the region could have been derived from the rejuvenated Neoproterozoic rocks rather than directly from the Mesoproterozoic metamorphic basement.(5)The Shunxi,Miaoxi,Daochang and Moganshan plutons are highly fractionated I-type granites and zircon U–Pb dating yield weighted mean 206Pb/238 U ages of 131 ~ 137 Ma.Zircon U–Pb dating yields weighted mean 206Pb/238 U ages of 132 ~ 137 Ma?130 ~140 Ma?134 ~138 Ma and 130 Ma for the Tingzishan,Liuchun,Shanchuanxiang and Huangfenwu composite rocks.In addition,the lithology and weighted mean 206Pb/238 U ages of zircon U–Pb dating are granite(130 Ma),granodiorite,syenite(132 Ma),porphyritic granite(132 Ma)and granite(130 Ma)for the Xianxia,Tangshe,Tonglizhuang,Yangshan and Gaoling plutons,respectively.Among lithologic units of one composite rock or between one composite rock and other one,the granite and porphyritic granite show low(La/Yb)N and negative Sr and Eu anomalies compared with the granodiorite,implying they formed in relative shallow depths.The whole-rock ?Nd(t)and zircon ?Hf(t)have small differences and Two-stage Nd model(TDM2(Nd))ages(1132 ~ 1330 Ma)and Two-stage Hf model(TDM2(Hf))ages(989 ~ 1548 Ma)are the relatively unified Proterozoic for these I-type granites.However,the Daochang and Gaoling plutons from the north region have relative old Two-stage Hf model(TDM2(Hf))ages(1618 ~ 2587 Ma and 1542 ~ 2048Ma).It means that the north region may have a relatively old basement than the south region,which fully shows the heterogeneity of the melting source area for continental lower crust.It is the heterogeneity that created a wide variety of I-type granites in the region.(6)Combined previous work,the growth and evolution of the continental crust from the Mesoproterozoic to the late Mesozoic in the Eastern Segment of the Yangtze Block is divided into the following four stages:(a)The island arc volcanoes were formed by subduction of oceanic crust from the Cathaysia block under the Yangtze block at ca.>1.0 Ga.(b)During the arc-continent collision and syn-collisional magmatism at 0.7-0.9 Ga,the pre-existing Mesoproterozoic island-arc crust could have melted to produce widespread igneous rocks with low ?Nd(t)and ?Hf(t)values,including some A-type granites and mafic rocks.(c)During the Jurassic period(ca.150 Ma),the subduction of the Paleo-Pacific plate were beneficial for the asthenospheric mantle upwelling and decompression melting that generated basalt.The thin crust and continental lithospheric mantle in the region were affected and the Neoproterozoic basaltic rocks were partial melting to generate the Chishi pluton.(d)During the Cretaceous period,subsequent slab rollback of the Paleo-Pacific plate make the region have strong extensional tectonic setting and the underplating of basaltic magma more intense.And then the Neoproterozoic intermediate and acidic rocks were partial melting to generate large-scale I-type granites.Among them,the Neoproterozoic A-type granite were partial melting to generate ~ 130 Ma A-type granites along the fault zone in the region.